The differentiation capacity of isolated subsets from normal mouse thymuses has been examined in vivo and in vitro. Intravenous (i.v.) experimental cell transfers using congenic mice, have previously determined that an immature, intrathymic subset of adult mouse thymocytes are dull Lyl+ (dLyl) cells lacking both Lyt2 and L3T4 cell surface expression. These dLyl cells are committed thymocyte progenitors with limited capacity for regeneration. Immature thymocyte precursors have also been demonstrated to exist in bone marrow by i.v. transfer, but this route appears to restrict the "re-homing" of more mature cells back to the thymus. We have now determined that several additional, more differentiated, subsets of thymic cells can be transferred intrathymically (i.t.). This includes at least an Lyt2+, L3T4+ blast population and the mature L3T4+ thymocytes. We are currently examining whether the mature Lyt2+ population is also transferred. Yet, not all "immature" thymocytes are capable of transfer and further differentiation. Attempts to transfer early fetal thymocytes (day 13-14 of fetal gestation) have been unsuccessful, whereas both Lyt2+, L3T4+ and dLyl cells, present at 16-day, yield donor-derived cells. We have also evaluated the estrogen-sensitivity of immature dLyl thymocytes, and the ability of limited pretreatment with estradiol to render recipient animals receptive for thymocyte precursors. We have determined that exogenous estradiol administration appears to selectively deplete the large subcapsular thymic blast population and permit entry for a limited repopulation of donor-derived cells. We have also examined the in vitro growth requirements and differentiation of dLyl cells using interleukin and mitogen stimulation. The immature dLyl population has a selective growth requirement for a combination of IL1 plus IL2, but fails to further differentiate into other thymic subsets after the initial conversion of about 25% of Lyl cells to Lyt2+,L3T4+, within 18-24 hrs.